Ion activity measurement device



Feb. 23, 1943. E. D. COLEMAN ION ACTIVITY MEASUREMENT DEVICE` originalFiled Jan. 2o. 1959 r///Lu f/f//4!////////////////////Arl Patented Feb.23, 1943 UNITED STATES PATENT OFFICE ION ACTIVITY MEASUREMEN? DEVICEEdwin D. Coleman, Maywood, Ill. I

Original application January 20, '1939, Serial No.

Claims.

This invention relates to devices for .the determination of ionactivity, such, -for example, as hydrogen ion concentration.

'I'his is a division of my application entitled "Ion activitymeasurement device, filed January 20, 1939, Serial No. 251,975.

Until comparatively recent years Athe measurement of pH has been alaboratory procedure requiring that the measuring electrode such as thehydrogen electrode be made up 4just prior to the determination. Themaking up of this electrode required unusual skill. With the advent ofthe glass electrode this situation was somewhat improved. but there havestill been many diiilculties attendant` upon common and routine use ofpH equipment by comparatively unskilled operators. A common type of suchelectrodes is that known as the bulb electrode which consists of anouter electrode bulb partly filled with a solution commonly consistingof hydrochloric acid and quinhydrone into which dips an inner electrode.It has been the practice in the past to seal the electrode by means of awax, closing the end of the electrode stem. Frequent diiculty has beenexperienced because of the instability of the solu- Ition within theelectrode shell and due to changes in the electrode structure with timewhich introduced increasing errors. These failures of the bulb typeglass electrodes may be attributed to a number of major causes, amongwhich may be listed: (l) change in the composition of the solution withwhich the bulb is filled; (2) change in potential of the innerelectrode; (3) electrical leakage between the inner electrode terminalsand points at some other potential; (4) failure of the glass bulb, and(5) pseudo potentials on the electrode stem which show up at differentdepths of immersion. Another diiliculty heretofore associated with glasselectrodes of this type is that of leakage from or to the conductorsleading from the glass electrode proper to the potential measuringdevice, and the necessity for extreme precaution to Iprevent suchleakage.

An important object of the present invention is the provision of a glasselectrode and ion measurement device which is free of these and otherobjectionable features and -Which embodies cer'- tain features ofimproved construction and com-K position.

Figure 1 is an elevation of a glass electrode embodying my invention;

Fig. 2 is an enlarged longitudinal section therethrough;

Fig. 3 is an enlarged section through the inner electrode. and Y Dividedand this application February 8, 1940, Serial N0. 317,806

Fig. 4 is a wiring diagram of the complete measurement device.

According to one phase of the present invenl tion, the electrodecomprises a stem designated generally by the numeral 3, of highresistance glass, preferably lead glass having a coeiiicient of thermalexpansion of about 9.2 106 vover the range 0-350" C., such as G12 madeby Corning Glass Works, Corning, New York, and capable of beingpermanently fused and sealed to a low resistance glass bulb 4 asindicated at 5. The low resistance bulb 3 is preferably formed of glassof a composition about as follows:

vPer cent CaO 6 NazO 22 one example of which is that known commerciallyas Corning 015. By constructing the outer shell in this manner, I obtaina highly improved result in eliminating the necessity of alwaysimmersing the electrode to a given depth in the solution under test. Byusing av high resistance stem in combination with a low resistance bulb,the pH sensitivity is substantially limited to the bulb portion of .theshell and the depth of immersion is not Vcritical so long as thebulbisvglass such vas described for the stem, and I am of the opinion thatthis breakage has in the past been due to a difference in the coecientof expansion of the two pieces of glass fused together at this point. Atleast the diculty seems .to be remedied by employing a bulb and a stemwhich are Within close limits of having the same coelii-v cient ofthermal expansion.

The bulb 4 and the lower end of the stem 3 are lled with a solution, inthis instance consisting of two tenths normal hydrochloric acid which issaturated at 4 C. with potassium chloride and saturated at operatingtemperature with silver chloride. While I have stated that the solutionis made from two tenths normal hydrochloric acid, and I prefer to keepit as close as possible to this concentration because of the bettertemperature correction thereby obtained as will presently be described,the critical range of concentration is from about one tenth normal toabout three tenths normal hydrochloric acid.

'I'he bulb and stem are filled with inner solution to about the levelindicated at 6, leaving a space 1 above the liquid for expansion of thesolution with temperature changes. An inner electrode 3 within the bulb4 is immersed in the solution 6 and is carried in a glass tube 3preferably of the above described lead glass, the electrode extendingthrough the lower end of this tube as shown at and sealed therein byfusion of the glass tube. A conductor i2 extends upward through the tube9 and out through the upper end thereof. The upper end of the stemprojecting end I4 is plated with a layer i5 ofv metallic silver, afterwhich the silver plate is made anode in a chloride solution and chlorinedeposited against the silver to react therewith and produce an outerlayer of silver chloride, herein designated at i6. It will beAunderstood that numerous different methods may be employed forproducing this structure, the method above described being merely`illustrative, the essential thing being to produce a' coating of silverchloride on a silver electrode surface.

'I'he composition of the inner electrode and of the inner solution issuch that the potential with respect to the inner surface of theelectrode shell will undergo the same increment with respect totemperature as will occur at the inner electrode at the same temperatureincrement. I have found by adopting the combination of inner electrodeand inner solution above described that -this can be caused to occurwithin reasonable so that the potential between the inner electrode andthe inner wall remains substantially constant with change intemperature.

The glass electrode proper is carried in a support, in this `instanceconsisting of 'a tube |1 shown in the drawings as ofinsulation-material, but which may also be of metal since its functionis that of a guard ring, said tube concentric with the stem 3 of theelectrode and spaced therefrom, the space between the tube |1 and thestem being filled with a thermo-plastic material i8, such as pitch,rosin, or othersemi-insulating thermo-plastic material. The tube I1preferably extends somewhat beyond the end of the electrode proper, andthe space therein is likewise filled with the thermo-plastic material asshown at i9. Beyond theend of the 'electrode y, proper and within thethermo-plastic material I9, the end sof the conductor I2 is electricallyconnected with a lead 2|, as shown at 22.

Referring now more particularly to Fig. 4, in service the glasselectrode is mounted in a frame or supporting structure herein generallyindicated at 23, the element 23 cooperating with the tube |1 to supportthe electrode in the required posltion. The lead 2| is enclosed by ashield 24, surrounding and spaced therefrom, the shield`consisting, inthis instance, of a tube of braided Wire, insulation 24a beingpositioned between the lead 2| and the inner surface of the shield 24.The outer surface of the shield is covered by a layer of rubberinsulation 24h. The shield 24 extends into the end of the tube l|1 andis held in position within the thermo-plastic material in the region i3.The end of the shield is preferably brought down into closely spacedrelation with the upper end of the electrode proper so that thesemi-insulation thermo-plastic material is excluded from` the regionbetween the upper end oi' the stem and the inner surface of the shield.This is preferable but not essential since the semi-insulation materialin contact with the stem 3, between this point and the bulb, itselfconstitutes an effective guard ring preventing movement of electrons tothe connection 2| and lead I2 along the outer surface of the electrodestem at the condition of balance. The

-shield 24 is Aconnected as indicated at 25 to what connection beingmade in any of the usual well known manners. The reference electrode,which may be an ordinary Calomel half cell, is connected by a conductor23 to the slider 3| of a potentiometer circuit, designated by the slider3| operating on a slide wire 32, having terminals 33 and 34 and beingfurther connected in series with a battery 35 and a variable resistance33. This potentiometer circuit may be standardized by any of the wellknown procedures, the standardizing circuit not being here indicated.Point' 33 of the potentiometer circuit is also carried to the commonMcClure point, as indicated at 31 by a conductor 38,. Now it will benoted that the potentiometer circuit has'between the points 3| and 33 apotential due to the battery 35 which is opposite in `sense to thepotential developed by the cell, comprising the reference electrode 21and the glass electrode, feeding conductor 2|, which potential isimpressed on the potentiometer circuit in a manner presently to bedescribed. By'properly adjusting the position of the slider 3| these twopotentials can be made equal and opposite, in which instance the point33 and the conductor 2| are at the same potential. There is also shownin Fig. 4 apparatus for determining when this condition of equality isattained by virtue of an impulse type amplifier, a device which willindicate any diiference in potential between the cell and potentiometerand which will show no indication when the condition of equality isattained. This may also take .the

form of anywell known type of potential detectors, such as theD'Arsonval galvanometer, plate electrometer, or the like.

The device for detecting the point of potential equality between thecell chain and the potentiometer includes a two point switch 33 having aterminal 4| connected to conductor 2| and terminal 42 connected to thepotentiometer slide wire 32 at 33. Terminal 42 further connects to thecathode 43 of a vacuum tube indicated generally by the numeral 30. 'I'heswitch 39 has a blade 44 movable to establish contact with either of theterminals 4| or 42 as required, and'is in turn continuously connected toa condenser 40 which is in turn connected to the grid 45 of the samevacuum tube. The plate 43 of said vacuum tube connects through meter 41and B batteries 48 to the cathode 43 and by conductor 49 to the point 33of the potentiometer circuit. The filament of said vacuum tube isexcited by an A battery In practice the blade 44 of switch 39 is movedfrom the position of rest in contact with terminal 42 into contact withterminal 4| and at the same time the operator observes tne action onmeter 41. Now if there exists any dinerence inl potential betweenterminals 4| and 42 there will occur a. movement of the needle of meter41, the direction and magnitude of this needle movement indicating howfar and in which direction the potentiometer is out of balance. Theoperator subsequently moves slider 3l toward the position of balance asthus indicated and again checks by moving switch blade-44 from contact42 to contact 4|, repeating adjustments of the potentiometer slider 3luntil the condition of equality is reached whereat points 42 and 4| areat the same potential and no movement of the meter needle occurs whenswitch blade 44 is moved alternately between terminals 42 and 4|. Atthis point of equality the potential developed in the potentiometercircuit is equal and opposite to that of the cell consisting of thereference electrode and the glass electrode. The usual calibrations 52on the potentiometer slider Wire then indicate either the potential ofthe cell or the pH of the solution being tested, depending upon theunits in which the scale is calibrated.

Now it will be noted that at the condition of equality conductors 2| andthe point 33 are at the same potential. Further shield 24 is at thissame potential since the shield and the point 33 are both at McClurepotential. Further, the support 23 for sleeve |1 is also at McClurepotential. As a result of this unique construction, it is impossible forleakage to occur from the conductor 2|, since it is exposed at no pointto potentials other than McClure. By utilizing such construction it isnot necessary to employ high insulation between the conductor and theshield 24, since current cannot flow unless difference of potentialexists.

While I have herein shown one form of device for measuring the potentialbetween the reference Aelectrode conductor 29 and the glass electrodeconductor 2|, the measuring device shown in Figure l of my copendingapplication Serial No. 252,779, filed January 25, 1939, may also beused.

The above described construction substantially eliminates thediillculties heretofore associated with glass electrodes of the bulbtype as has been shown through prolonged investigation and extensive usein the ileld. I have discovered that the reason for the depreciation ofprior art electrodes is in part found in the manner in which theelectrode stem was sealed. These are commonlysealed with a wax-likesubstance, such as shellac, petroleum pitches or sealing wax, and

that while electrodes made in this manner operate satisfactorily for ashort periodof time,

- the nature of the seal soon introduces inaccuracies in the result forthe reason, first, that all causing a material change in the potentialof concurrent change at the inner surface of the the inner electrode;secondly, when wax or pitch to glass surfaces are exposed to the innersolution for extended periods, the pitch becomes saturated with a fluidof different chemical compo sition than the inner solution.Consequently, if the inner electrode or any metal electrically connectedto the inner electrode is allowed to contact the saturated sealingmaterial, a potential is established and galvanic action occurs betweenthis region of contact and the inner electrode proper, these pointsforming the two electrodes of a galvanic cell short circuited by theinner electrode. The galvanic action results in eventual failure of theinner electrode; thirdly,

the saturated sealing compound tends to separate from the glass allowinga thin film of the inner solution to follow along the region ofseparation and establish electrical contact with points outside theelectrode bulb which points may be at other potentials. Underthesecircumstances a very large error occurs. Change in potential of theinner electrode may result from one of two reasons: ilrst, residualgases within the sealed bulb may cause the formation of surface films onthe electrode which have a different potential from that of the originalsurface itself. The formation of lead oxide over lead is an example;second, the character of the solution itself may alter .causing theinner electrode potential to undergo a concurrent change. For example;if quinhydrone is employed, change in the quinone to hydroquinone willcause a change of potential of the inner electrode without a glass bulb.

On the other hand, with my improved construction the .conductor I2 iscompletely insulated from the inner solution 6 at all points, and at thesame time the solution is hermetically sealed in place, in glass, andcannot evaporate,

become contaminated or cause conductance between the inside and outsideof the bulb by other than the desired path through the bulb 4. Themetal-to-glass seals are permanent. The solution is sealed against airso that there is no oxidation or other chemical action on the solution.

While I have described the electrode as being a glass electrode, it isknown that materials other than glass, for example, quartz and wax alsoexhibit substantially the same potential response to pI-I as does glass,and in the use of the term glass as applied to certain phases of thisapplication, I refer to any material other than glass which may be usedfor this purpose.

I claim:

1. The combination in a potential measurement device of the type inwhich a source of potential may be located at a remote point from ameasuring means of a high impedance source of potential, a potentiometercircuit for measuring the potential thereof, means -for insulating thement device of the type in which a source of potential may be located ata remote point from a measuring means of a high impedance sourcecomprising an electrode having a stem, a measuring means for measuringthe potential therei' adapted to be remotely placed with respect to saidelectrode, a lead `connecting said electrode and said measuring means,means receiving said stem for mounting said electrode, an

for maintaining said mounting, said shield, and said lead at the samepotential at the instant of measurement.

3. The combination in a potential measureyment device of a glasselectrode comprising an electrode proper having a stem, an electrodemounting sleeve around said stem, insulation material interposedbetween'said stem and sleeve vfor mounting the stem therein,` ameasuring means, a lead connecting said stem and said measuring means,an electrostatic shield enclosing said lead and insulated therefromhaving .electrostatic shield for said lead insulated therefrom andconnected to said mounting, and means one end embedded in saidinsulation material and enclosing said lead within said mounting sleeve,and means i'or maintaining said shieldv and said lead at the samepotential at the instant of measurement.

tential source with said measuring circuit, anA

electro-static shield for said conductor insulated thereirom, and meansincluding a potential source for maintaining said shield and saidconductor at the same potential. at the instant of measurement.

5. The combination in a potential measurement device of a glasselectrode including a stem and a conductor extending theretrom, anelectrode mounting sleeve around said stem, semi-insulation materialinterposed between said stem and sleeve for mounting the stem therein, ameasuring means, a lead connecting said conductor and said measuringmeans, and means for maintaining said semi-insulation material 'and saidlead at the same potential at the instant of measurement.

EDWIN D. COLEMAN.

